Sams Teach Yourself JavaScript in 24 Hours

By Michael Moncur

• Table of Contents
  • Copyright
  • About the Author
  • Acknowledgments
  • We Want to Hear from You!
  • Reader Services
  • Introduction
  • Part I: Getting Started
  • Hour 1. Understanding JavaScript
  • Hour 2. Creating a Simple Script
  • Hour 3. How JavaScript Programs Work
  • Part II: Learning JavaScript Basics
  • Hour 4. Using Functions and Variables
    • Using Functions
    • Understanding Expressions and Operators
    • Using Variables
    • Data Types in JavaScript
    • Converting Between Data Types
    • Workshop: Storing User Data in Variables
    • Summary
    • Q&A
    • Quiz
    • Exercises
  • Hour 5. Using Strings and Arrays
  • Hour 6. Testing and Comparing Values
  • Hour 7. Repeating Yourself: Using Loops
  • Hour 8. Using Math and Date Functions
  • Part III: The Document Object Model (DOM)
  • Hour 9. Working with the Document Object Model
  • Hour 10. Responding to Events
  • Hour 11. Using Windows and Frames
  • Hour 12. Getting Data with Forms
  • Hour 13. Using Graphics and Animation
  • Part IV: Moving on to Advanced JavaScript Features
  • Hour 14. Creating Cross-Browser Scripts
  • Hour 15. Creating Custom Objects
  • Hour 16. Working with Sounds and Plug-Ins
  • Hour 17. Debugging JavaScript Applications
  • Part V: Working with Dynamic HTML (DHTML)
  • Hour 18. Working with Style Sheets
  • Hour 19. Using Dynamic HTML (DHTML)
  • Hour 20. Using Advanced DOM Features
  • Part VI: Putting It All Together
  • Hour 21. Improving a Web Page with JavaScript
  • Hour 22. Creating a JavaScript Game
  • Hour 23. Creating DHTML Applications
  • Hour 24. JavaScript Tips and Tricks
  • Part VII: Appendices
  • Appendix A. Other JavaScript Resources
  • Appendix B. Tools for JavaScript Developers
  • Appendix C. Glossary
  • Appendix D. JavaScript Quick Reference
  • Appendix E. DOM Quick Reference

Data Types in JavaScript

In some computer languages, you have to specify the type of data a variable will store: for example, a number or a string. In JavaScript, you don't need to specify a data type in most cases. However, you should know the types of data JavaScript can deal with.

These are the basic JavaScript data types:

• Numbers, such as 3, 25, or 1.4142138. JavaScript supports both integers and floating-point numbers.
• Boolean, or logical values. These can have one of two values: true or false. These are useful for indicating whether a certain condition is true.

  Note

  You'll learn more about boolean values, and about using conditions in JavaScript, in Hour 6, "Testing and Comparing Values."

• Strings, such as "I am a jelly doughnut". These consist of one or more characters of text. (Strictly speaking, these are string objects, which you'll learn about in Hour 5, "Using Strings and Arrays.")
• The null value, represented by the keyword null. This is the value of an undefined variable. For example, the statement document.write(fig) will result in this value (and an error message) if the variable fig has not been previously used or defined.

Although JavaScript keeps track of the data type currently stored in each variable, it doesn't restrict you from changing types midstream. For example, suppose you declared a variable by assigning it a value:

total = 31;

This statement declares a variable called total and assigns it the value of 31. This is a numeric variable. Now suppose you changed the value of total:

total = "albatross";

This assigns a string value to total. JavaScript will not display an error when this statement executes; it's perfectly valid, although it's probably not a very useful total.

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